The hypocycloidal principle involves making a gear wheel “walk” round the inner circumference of a ring gear, or vice-versa, thus imparting a rotation to the other member that is counter to the direction of oscillation. Commercially available groups of reducers use a similar principle. The most common is the harmonic wave drive which uses a flexible geared spline which is pressed by an inner wave generator at opposite points against an outer fixed gear ring, thereby transferring rotation to the flexspine at high reduction ratios. The output rotates contrary to the input, which is advantageous because it reduces the reaction force required to be supplied by the mount. The layout is that an inner dual pressure wave input forces a flexible end of a cup shaped transfer ring against a fixed outer ring. Output is taken from the rigid bottom of the cup and is in a contra direction to the input. However, this type of reducer is expensive, the flexspine is subject to fatigue failure and it cannot be made continuously variable.
It is also known to use a non flexible hypocycloidal principle whereby the input is divided between two or more eccentric cams rotating within a ring having internal gears and causing it to wobble round an inner gear and so impart rotation to it. The layout is that multiple eccentrics rotate in holes in a ring with internal gears causing the ring to oscillate round an inner output gear, producing rotation contra to input direction. This design has low torsional wind up, but is complex and sliding of the eccentric cams make it inefficient.
A further type of hypocycloidal drive sometimes called a quadrant drive uses a cam on the input to cause a gear to wobble and roll round within a ring gear from which output is taken. The layout comprises an eccentric central input which rotates within a central gear wheel causing it to oscillate against the geared inner side of a fixed ring gear. The reaction produces a contra rotation in the oscillating gear. A further eccentric stage cancels the oscillation to produce simple rotation.
A further group is the cyclo drive. It employs an eccentric inner input to oscillate a central disc, which is restrained by outer rollers. The output means has roller pins, which extend into holes in the oscillating disc. Output is contra to input. This is essentially the same layout as the quadrant drive except that the eccentric output is cancelled by the roller pins on the output, rather than by a separate stage. This design is inherently expensive because of the large numbers of components and roller bearings.
The invention is set out in the claims. The invention provides a less complex and expensive layout than known approaches, can be employed in both friction and positive embodiments and can be used in both fixed ratios and continuously variable embodiments. Further advantages are set out in the following description.